Ueda said that the ending was open to interpretation, so I think it's really up to us to decide if Trico survived. Trico was badly wounded, but I like to think that the wounds weren't fatal.
Did you guys find the transparent green spheres in one of the rooms (the room containing the chain with the barrel on it)? I wondered if these were eggs left behind by Trico's kind. Another theory I had is that the butterflies we see everywhere are baby Tricos, of which only a few would survive and grow into adults. They are attracted the barrels the same way that Trico is. We don't see much other life in the valley so why would only the butterflies continue to exist in such great numbers?
Warning, Ico/Shadow of the Colossus spoilers follow...
There are a lot of thematic similarities between this game and Ico/Shadow of the Colossus. Particularly the use of light and dark. I am wondering if the stories are connected. We know that Shadow of the Colossus is prequel to Ico. If The Last Guardian is part of the same continuity, when would it take place? After Ico? Before Shadow? We do not have much to go on.
At first I thought that Trico might be a colossus since they come in all shapes and sizes, but the appearance of the colossi all have a distinct features that unite them that are missing from Trico. So I'm thinking that Trico's kind could be a kind of a colossus-like being, but one that was based on an original creature, whereas I had assumed (but perhaps wrongly) that the colossi were created for the particular purpose of housing Dormin's soul. But whether Trico's kind are colossi or colossus-like, it's in their use that they are similar: someone has used the body of another creature for their own purposes. The master of the valley consisted of a similar dark energy to Dormin and the Queen from Ico and the master uses the bodies of others (Trico's kind and the humans brought into the valley) as a 'life' source. I love the contrast here: your relationship with Trico is the very opposite of the relationship between the master and his minions, one that exists in spite of a lack direct control, and it's awesome that this contrast is communicated directly through gameplay. We don't know much about the master of the valley who is more of a lifeless antagonist than the other two, perhaps again to directly contrast with Trico who is full of life. Even the suits of armour appear to be empty, furthering the theme of lifeless control. Perhaps the master is simply representing the amassing of this dark, lifeless energy in a single location, rather than being an autonomous being. The master might simply be a literal manifestation of death who feeds on life to survive.
Another question. What does the name The Last Guardian mean anyway? I get that the 'guardian' part is referring to Trico being a guardian of the boy. But why the last? I'm wondering if this could be a reference to the series as a whole. The colossi were guardians of the seal of Dormin. Once this is broken, Wander becomes a colossus and he dies, but part of him passes on to the baby with horns, who is assumed to be an ancestor of Ico. Then, Ico becomes the guardian of Yorda. Finally, it's not clear how it is connected, but we have Trico, who bears a similar name and the horns of Ico, who is the last guardian. I am not sure that there is a physical link here, but it seems that there is at least a thematic one. The last guardian in a line of guardians? The last guardian in a trilogy? I list Trico here, where Ico and Wander were the original two. Although the boy is the playable character of The Last Guardian, I see Trico as the true hero of the story; the roles from Ico have been swapped around. Interestingly the boy in The Last Guardian was originally going to be a girl, which would bring the parallel with Yorda as the one to be guarded, but I'm glad they changed him to be a boy to avoid damsel in distress and beauty and the beast type cliches. Anyway, it may be that the last guardian title has no direct story link with the previous games... or it could mean that Trico inherited the role from the boys with horns directly, perhaps by eating one of them while under the control of the master.
It does seem that the valley was constructed for a particular purpose, and that there's more to the valley than simply being a control centre for the master. Why would the master keep the shield in the valley, including pedestals that directly interact with it? Everything about it is contrary to the dark energy of the master. By guess is that an ancient civilisation created the walled valley to keep the master in, similar to how the forbidden land and the colossi were cut off from the rest of the world to keep Dormin safely away and likewise the castle in Ico appeared to keep the Queen in (she says that Yorda can never leave, since she is her daughter, so it seems she couldn't leave). In keeping with these themes, perhaps someone like Lord Emon the shaman from Shadow of the Colossus created the valley stronghold to keep the master in. Shamans often carried a shield to defend against evil spirits, and the master could surely be considered one. After the shamans were long gone, Trico's kind then migrated into the valley naturally and were caught by the master's armoured guards. We see near the end that the white wall slots where the suits of armour come from are lit up by using the shield. It appears that the entrance way to the master (through the ceiling) required the use of light to ensure that no dark forces could break the master out.
I'll post more thoughts as they come to me. It's all very interesting and thought provoking material, and I'm sure that there's a lot that we've missed.